THE HABITAT SATURATION HYPOTHESIS AND SOCIALITY IN AN ALLODAPINE BEE - COOPERATIVE NESTING IS NOT MAKING THE BEST OF A BAD SITUATION

Several factors thought to be important for the evolution of cooperati ve breeding in vertebrates have received little attention in facultati vely social insects. One of these, the ''habitat saturation hypothesis '' of Selander (1964), predicts that colony sizes will be greater in b reeding units where dispersal opportunities are limited, suggesting th at group living is a secondary option to independent reproduction. The Australian allodapine bee Exoneura bicolor exhibits a number of trait s that occur in cooperatively breeding bird species, including long li fe-span, repeated opportunities for reproduction, and vulnerability to brood predation and parasitism. We experimentally examined the effect of a potentially limiting environmental factor, nesting substrate ava ilability, as an agent influencing sociality in E. bicolor. We manipul ated nesting substrate availability in two separate locations during a time when foundress dispersal is common. No significant difference wa s found between colony sizes in cases where dispersal options were abu ndant and cases where dispersal options were limited. An increase in o pportunities for dispersal did not lead to higher rates of independent nesting, suggesting that cooperative nesting is a preferred strategy regardless of distance-related costs of dispersal. Reproductivity per female and brood survival were examined as factors selecting for group living. Low survival of brood in single-female nests has the potentia l to select for cooperative nesting in this bee.